Chemical vapor deposition (CVD) is widely used in the semiconductor industry to deposit films of various kinds, such as intrinsic and doped amorphous silicon (a-Si), silicon oxide (SixOy), silicon nitride (SirNs), silicon oxynitride, and the like on a substrate. Semiconductor CVD processing is generally done in a vacuum chamber by using precursor gases which dissociate and react to form the desired film. In order to deposit films at low temperatures and relatively high deposition rates, a plasma may be formed from the precursor gases in the chamber during the deposition. One type of such plasma processes is plasma enhanced CVD (PECVD). Another type of such plasma processes is high density plasma CVD (HDP-CVD).
Many processing chambers are made of aluminum and include a support for the substrate and a port for entry of the required precursor gases. When a plasma is used, the gas inlet and/or the substrate support is connected to a source of power, such as a radio frequency (RF) power source. A vacuum pump is also connected to the chamber to control the pressure in the chamber and to remove the various gases and contaminants generated during the deposition.
In electronic device processing, it is desirable to keep contaminants in the chamber to a minimum. During the deposition process however, the film is deposited not only on the substrate, but also on walls and various components, e.g., shields, the substrate support and the like, within the chamber. During subsequent depositions, the film on the walls and various components can crack or peel, causing contaminants to fall on the substrate. This causes problems and damage to particular devices on the substrate. Damaged devices have to be discarded.
When large glass substrates are processed, for example, to form thin film transistors for use in flat panel displays and the like, more than a million transistors may be formed on a single substrate. The presence of contaminants in the processing chamber can be even more problematic in this case, since the flat panel display is likely to be inoperative if damaged by particulates. In this case, an entire large glass substrate may have to be discarded.
Thus, processing chambers are periodically cleaned to remove accumulated films from prior depositions. Cleaning may be performed by passing an etch gas, for example a fluorine-containing gas, such as nitrogen trifluoride (NF3), into the chamber. A standard method of performing this cleaning procedure is to pass a constant flow of NF3 into the chamber. A plasma is initiated from the fluorine-containing gas which reacts with coatings from prior depositions on the chamber walls and fixtures, e.g., coatings of Si, SixOy, SirON and the like, as well as any other materials in the chamber. In particular, the NF3 creates free fluorine radicals “F*” which react with Si-containing residues.
Alternatively, many electronic device processing chambers, use a remote plasma cleaning system (RPCS) for removing residual accumulation from the inside of the chamber after substrate processing. During cleaning, a remote plasma source (RPS) is coupled to the processing chamber and plasma (e.g., a fluorine-containing gas plasma such as NF3) is fed into the chamber to react with the residues from prior depositions. The resulting gases are then pumped out of the chamber via an exhaust outlet.
The frequency and duration of a cleaning cycle were typically determined by trial and error or using historical data. For instance, a chamber may be scheduled for cleaning after processing a predetermined number of substrates, regardless of the condition of the chamber. With respect to duration, it can be difficult to accurately determine when the cleaning has completed. To insure that the chambers are thoroughly cleaned, an extra 20 to 30 percent of the expected cleaning time may be added to the cleaning cycle, without regard to considering the damage that the extra cleaning time may cause to the chamber and the components contained therein. Thus, what is needed are improved methods and apparatus for detecting the cleaning end point.